Warring forces in South Sudan have carried out horrific crimes against children, including castration, rape and tying them together before slitting their throats, the UN has said.

"Survivors report that boys have been castrated and left to bleed to death... girls as young as eight have been gang raped and murdered," UN children's agency chief Anthony Lake said in a statement released earlier this week.

"Children have been tied together before their attackers slit their throats... others have been thrown into burning buildings."

Tens of thousands are believed to have been killed in the 18-month war, although there is no clear toll. At least 129 children were killed in May in the northern state of Unity, scene of some of heaviest fighting in the civil war, Unicef added.

Children rush to a field demarcated for food-drops on February 24, 2015 at the village in Nyal in South Sudan's, Panyijar county.

Civil war began in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings across the country that has split the poverty-stricken, landlocked country along ethnic lines.

It has been characterised by ethnic massacres, rape and the use of child soldiers.

"The violence against children in South Sudan has reached a new level of brutality," Lake added. Thousands of children have also been abducted to fight.

"Children are also being aggressively recruited into armed groups of both sides on an alarming scale – an estimated 13,000 children forced to participate in a conflict not of their making," Lake added.

"Imagine the psychological and physical effects on these children – not only of the violence inflicted on them but also the violence they are forced to inflict on others."

A quarter of a million children face starvation , while two-thirds of the country's 12 million people need aid, with 4.5 million people facing severe food insecurity, according to the UN.

"In the name of humanity and common decency this violence against the innocent must stop," he said.